The conventional orthodontic appliances consist of an orthodontic band which is totally circumposed about every tooth and a bracket which is usually affixed by welding. The typical, present-day use of such apparatus requires the orthodontist to invest in a very large inventory of bands which are of proper anatomical design and size to fit every tooth. It is not unusual to require as many as 30 separate band sized for each type of tooth in order to obtain this desired aim. Furthermore, the ability of the orthodontist to correctly band the tooth depends upon his adhesive bonding of this band and its remaining in place throughout the entire treatment period without failure or without dental caries occurring under the band. It is further present-day practice to have the brackets prewelded to the bands and, of course, this increases the inventory of the orthodontist.
In the recent past, a variety of techniques have evolved for directly attaching orthodontic brackets to the surface of teeth. Of the methods in use, the first type utilizes orthodontic brackets molded from thermoplastics such as polycarbonate. These brackets are directly bonded to the surface of the teeth by means of an adhesive system which usually consists of a powder composed of an acrylate or methacrylate polymer and benzoyl perioxide, and a liquid monomer.
Each of these types of adhesives in use today require the mixing of this powder and liquid this is usually accomplished by means of a scoop for measuring the powder and an eyedropper for dispensing the liquid. This mixture is mixed with a spatula for a specific time period. These means of measurement are very crude and it is very difficult to control the time and uniformity of the mixing to provide optimum strength. Furthermore, in the bonding of each bracket, it is necessary to mix another batch of powder and liquid. The sources of error from this type of procedure are obvious since with a large excess of powder and insufficient liquid, there is a decrease in the strength from that of the optimum mix and the same is true if there is an excess of liquid over powder.
In addition, it may be noted that the time in which the adhesive polymerizes is a function of how much powder is used in the mixture. A further complication of this type of adhesive is that it must be sufficiently fluid that it will adequately wet the enamel rods of the enamel surface. When the mixture is sufficiently fluid to obtain this goal, the bracket containing the adhesive must be held in place for between one and two minutes until sufficient strength has been obtained to assure that the bracket will remain where it was positioned.
A further deficiency of this system is that polycarbonate brackets are insufficiently strong to withstand the intra-oral forces. Furthermore, the brackets are notoriously prone to creep under the load and they become scratched and discolored after a few months in use. This problem is particularly apparent with Edgewise techniques since an important requirement is to maintain an accurate rectangular slot throughout treatment and this is impossible with these brackets since they deform so much that the dimensional characteristics of the rectangular slot are degraded within a matter of a few days.
A variety of techniques have been described and are in use in direct bonding stainless steel brackets to teeth. These all depend upon mechanical interlocking of the bracket to the tooth. Examples include brackets welded to wire mesh through which the adhesive permeates, bracket bases with holes around the perimeter or throughout the bracket base through which the adhesive must be extruded, or box-shaped devices into which the adhesive must be placed and enclosed and mechanically interlocked.
The principal deficiency of wire mesh is that it must be of sufficient size to have strength on the weldments, otherwise they lack the strength required for orthodontic treatment. A further deficiency is that the wire mesh is unsightly and must be covered by the adhesive and since the adhesive mesh interface is rough, it serves as a site where cariogenic bacteria and food debris can accumulate. The bracket bases with holes around the periphery or throughout the entire base are only attached by the adhesive extruded through these holes. All that is required for failure of the bond is for mastication forces to shear the adhesive rods penetrating these holes. This comprises only a small fraction of the total area of the bracket base and, therefore, such bases have less strength than a base would have should all of the surface of the base be participating in bonding strength. In order for this type of bracket base to be cosmetically acceptable, it is necessary to feather out the adhesive which is extruded through these holes to form a surface which will cover what would otherwise be an unsightly area. This is a very time-consuming process. This type of bracket has a higher profile on the tooth and has higher torque forces in mastication.
An additional technique for direct bonding brackets to teeth consists of utilizing ultra violet light as a catalyst. In this type of system, polycarbonate brackets have been used extensively and recently this method has been applied to metal brackets which have bases with holes around the periphery. The major deficiency with this system is that it is necessary to hold an ultra violet light within 1 to 2 milimeters of the bracket and move it around the entire periphery of the bracket throughout the process of catalysis. This requires between one and two minutes for each bracket. Again, this process is time consuming and may even be dangerous, since there is insufficient history of the effects of ultra violet light on the viable tissues of the mouth.
One of the major causes of concern during orthodontic treatment is decalcification under the bands or brackets. The presence of wires and orthodontic appliances tend to entrap food debris, cariogenic bacteria thrive, and it requires diligence on the part of the patient in oral hygiene to prevent dental caries. Frequently beneath the brackets and bands this decalcification can take place and not be noticed until orthodontic treatment is completed and upon removal of the devices, such decalcification can be discovered. This is, of course, catastrophic. A method of preventing this has been in use in which a sealant is catalyzed by the ultra violet light previously described for the direct bonding of orthodontic brackets. The deficiencies of catalyzing the sealant with ultra violet light have been previously noted. Other types of sealants are in use for this purpose which unfortunately do not set up effectively because they are inhibited by air.